Field of the Invention
The invention relates to a method for operating a magnetic resonance facility for acquiring image data of a patient in a manner defined by acquisition parameters, the acquisition of the image data being preceded by the performance of at least one alignment operation for adjusting the operating parameters of the magnetic resonance facility that have an influence on the acquisition conditions for the current patient. The invention also relates to a magnetic resonance facility that is operated by such a method.
Description of the Prior Art
Magnetic resonance imaging has become a widely used standard, particularly in the field of medical engineering, as it allows a number of patients to be examined in very different ways. Depending on the purpose of the examination, there are a number of configuration and selection options for acquiring image data using a magnetic resonance facility. For example, it is possible to select defined magnetic resonance sequences to set a defined, desired, underlying contrast; a number of further acquisition parameters, which describe the actual acquisition operation, allow the acquisition process to be optimized in different magnetic resonance protocols for defined clinical questions and examination regions to be recorded.
A magnetic resonance facility requires the most homogeneous magnetic field possible in the imaging region. The introduction of patients can cause the previously homogeneous magnetic field to become distorted, leading to deterioration of the image quality. Also different transmitter voltages, which define the strength of the radio-frequency excitation to be brought about by radio-frequency pulses in the examination region, have to be selected for different patients, particularly with respect to their spread and/or limit values for electromagnetic exposure (SAR values). In order to adjust the operating parameters of the magnetic resonance facility defining acquisition conditions for specific patients who are to be examined, it is known to perform alignment operations before the actual acquisition of the image data.
To this end it is known, for example, to acquire B0 and B1 field maps, in other words basic magnetic field maps and/or radio-frequency magnet field maps, while a patient is already positioned for the acquisition, in order to improve shim measures for increasing the homogeneity of the respective magnetic fields. It is also known to perform an alignment operation with respect to the transmitter voltage. Alignment operations are also known with respect to the frequency of the radio-frequency excitation. In other words, patient-specific alignments are used to adjust the frequency, the basic magnetic field, and the transmitter voltage as precisely as possible for ideal acquisition conditions, for example a homogeneous B0 field. Operating parameters can be shim parameters, lower and/or upper limits for the transmitter voltage, etc.
After the alignment operations have been completed, specific effects of the patient are generally no longer considered. The magnetic resonance sequences or magnetic resonance protocols are generally designed to be so robust that expected variations in the acquisition conditions for different patients do not result in image quality degradation as far as possible. However, this is at the expense of acquisition efficiency and/or image quality. It is known for a user to perform patient-specific adjustments manually, but this becomes more time-consuming as magnetic resonance acquisition methods become more complex and an extensive understanding of the relationships is required, because multiple acquisition parameters can be set manually and can also interact with one another with respect to their impact. The operators of magnetic resonance facilities have to undergo extensive training and it takes a great deal of time to perform such patient-specific adjustments manually. It is also a problem that the adjustment options for the operator are limited to the acquisition parameters that can be accessed, because not all conceivable acquisition parameters, or those used, can be set, for example via operator's user interface. Finding an optimum solution manually by adjusting acquisition parameters is also generally no longer conceivable, because there is a complex, multidimensional parameter space, which no longer allows a sufficiently good overview.
It is known with parallel transmission (pTX) of the radio-frequency excitation pulses to calculate the pulse shape as a function of B0 and B1 maps determined during an alignment, and this is essential with pTX, because defined geometric distributions of the excitation have to be achieved during parallel transmission.